Conventionally, development of measurement methods for the purpose of, for example, diagnosing various diseases by measuring a subject's eye movement has been advanced. For example, with the apparatus described in the following patent document 1, a subject's eye image obtained by a camera is observed as a target for changes in density value in the up-down direction, and an eyelid position is measured by using that the density value changes between the eyelid and eyeball. Also, with the apparatus described in the following patent document 2, a subject moving image is obtained by a camera, and an edge line to serve as a candidate for a combination of the upper and lower eyelids is extracted based on the brightness/darkness of the moving image, and with the apparatus described in the following patent document 3, a one-dimensional image showing a gray scale variation is extracted based on a luminance distribution from a subject's eye image, and a boundary point between the eyelid and eyeball is detected based on the one-dimensional image. These apparatuses all detect an eyelid position by processing a subject's image.
On the other hand, with the apparatus described in the following patent document 4, the intensity of reflected light and scattered light of light that are irradiated a subject's eye with is detected by a sensor to thereby generate a signal indicating whether the eyelid is in an open state or a closed state. This apparatus includes a detection device incorporated with an emitter and a sensor, and discriminates whether the eyelid is in an open state or a closed state by using that return light is weak at the surface of an eye and return light is strong at the surface of an eyelid.